The objective of this proposal is a quantitative study of urea production and excretion by the sheep fetus. Ewes with chronically indwelling catheters in the fetal and maternal circulation will be studied. The fetal urea production rates during periods of normal feeding and starvation will be determined. The placental clearances of urea and antipyrine will be measured simultaneously. The data of these measurements will be used to calculate placental permeability to urea at different stages of gestation. Data on placental permeability will be combined with data on placental weight, placental DNA content, fetal length and weight in order to describe the relationship of changes in placental permeability to placental and fetal growth in normal and growth retarded fetuses.